Monitoring of sleep state and sleepiness is useful in a clinical context for assessing subjects with suspected obstructive sleep apnoea, and in an industrial context for monitoring sleepiness in vehicle drivers and in actual work environments, particularly where sleepiness-related accidents can have serious consequences.
Obstructive sleep apnoea syndrome (OSAS) is a prevalent but under-diagnosed condition with serious cardiovascular consequences, and is highly treatable. Standard in-laboratory testing for OSAS involves an overnight study to assess severity of apnoea and sleep fragmentation, and may also involve an all-day study to assess daytime sleepiness. Assessment of sleep fragmentation can be achieved by analysing the overnight study to produce a chart of sleep stages, and the assessment of daytime sleepiness can be accomplished by measuring on multiple occasions over a day, the time taken for subjects to fall asleep (the so-called Multiple Sleep Latency Test).
However, due to increasing public awareness of OSAS and resource constraints in healthcare systems, in-laboratory testing for OSAS suffers from low availability in many countries. Therefore there is considerable interest among the professional societies in sleep medicine in the development of reliable low-cost techniques suitable for use in the home environment for identification of subjects with OSAS.
In the general population, the problem of sleepiness is recognised as a contributing factor to occupational and vehicular accidents. In the area of transportation safety, there is a lot of interest in methods to monitor driver fatigue. At the same time, there is interest in techniques that can complement existing in-laboratory clinical tools by assessing sleepiness over prolonged periods in the actual work environment.
A variety of techniques have been disclosed in the background art for addressing the need for sleep state and sleepiness monitoring outside of the laboratory environment.
For sleep state monitoring, portable systems incorporating the required signals for standard in-laboratory monitoring (2 channels of electro-encephalography, 2 channels of electro-oculography and 1 channel of electro-myography) have been developed. However, these systems require substantial clinical support to operate in the home environment. Subjects usually have to visit the sleep clinic before and after the overnight study to have the equipment set-up and removed. They also require the attachment of electrodes to the head, which can be disruptive and inconvenient.
Another method is to use a subset of the standard signals (such as a single EEG channel) to obtain an approximation. However, EEG monitoring at home requires significant user expertise, and it does not allow concurrent apnoea detection. Methods using other signals to provide estimates have also been proposed. These methods include actimetry, ECG plus respiration, and peripheral arterial tone plus actimetry. However, actimetry alone can only distinguish between wake and sleep but not the different stages of sleep, and also does not allow concurrent apnoea detection. Peripheral arterial tone monitoring requires proprietary and costly hardware and associated software. Respiration monitoring may be prone to sensor displacement in a home environment.
For sleepiness monitoring outside laboratory conditions, methods using eyelid closures, head movements, video surveillance, EEG, and actimetry have been proposed. However, methods using eyelid closures, head movements, video surveillance are not portable, so while it is feasible for applications such as driver sleepiness monitoring, it is not suitable for applications requiring ambulatory monitoring. EEG is ambulatory, but technical difficulties for long-term monitoring exist, and again it is inconvenient to have electrodes attached to the scalp. Sleepiness can be more objectively assessed with measurements such as the Psychomotor Vigilance Test (PVT) which measures a subject's reaction times. It has been shown that in a given subject, the reaction times will decrease as the person becomes sleepier—therefore PVT scores are used as a surrogate for sleepiness. However, PVT scoring takes approximately 10 minutes to carry out, and requires the subject's active attention so cannot be used for in-task monitoring of sleepiness.